---
title: "Why transition metal ions are coloured"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:tm-colour"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/tm-colour
site: BCT Engineering
---

# Why transition metal ions are coloured

Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications

- Asked in: 2073 Bhadra, 2.5 marks; 2073 Magh, 3 marks; 2070 Magh, 2.5 marks; 2068 Bhadra, 2 marks; 2068 Magh, 2.5 marks (asked 5 times)
- Video: [Colour of transition metal ions | the d-block elements | Chemistry | Khan Academy](https://www.youtube.com/watch?v=DsN6rSmpGOY) — Khan Academy India - English
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/tm-colour
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Why are most of the transition metal ions coloured in solution?

## Short answer

Ligands split the degenerate d orbitals into two energy levels. In ions with partly filled d subshells, electrons absorb visible light to undergo d–d transitions, and the complementary colour is transmitted. Ions with d⁰ or d¹⁰ configurations are colourless.

## Answer

Most transition metal ions are coloured because of **d–d transitions**.

1. In a compound or solution the ion is surrounded by ligands (water molecules), which split the five degenerate d orbitals into two sets of slightly different energy (t₂g and e_g in an octahedral field).
2. If the d subshell is partly filled, an electron can absorb a photon of visible light and jump from the lower (t₂g) to the higher (e_g) level. The energy gap Δ is of the order of visible light.
3. The ion therefore absorbs some wavelengths from white light; the remaining **complementary colour** is transmitted and seen.

```text
 e_g  ─── ───            ↑  electron excited
        ↑ hν (visible)   │
 t₂g  ─── ─── ───        │ Δ
 (d–d transition: absorbed colour → complementary colour seen)
```
Examples: [Cu(H₂O)₄]²⁺ blue, [Ni(H₂O)₆]²⁺ green, [Ti(H₂O)₆]³⁺ purple, [Cr(H₂O)₆]³⁺ violet, [Co(H₂O)₆]²⁺ pink, [Mn(H₂O)₆]²⁺ pale pink.

Ions with d⁰ (Sc³⁺, Ti⁴⁺) or d¹⁰ (Zn²⁺, Cu⁺) have no d–d transition and are colourless. Ions such as MnO₄⁻ and CrO₄²⁻ are coloured by charge transfer.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What causes the colour of most transition metal ions?
   A. d–d transitions
   B. Nuclear transitions
   C. Ionic size
   D. Radioactive decay
   Answer: A. Electrons absorb visible light and jump between split d levels. (Key point: Cause)

2. In an octahedral field, the five d orbitals split into
   A. s and p sets
   B. t2g and eg sets
   C. Two equal sets of three
   D. Seven f orbitals
   Answer: B. Ligands split the d orbitals into lower t2g and higher eg. (Key point: Splitting)

3. The colour seen is
   A. The colour absorbed
   B. Always white
   C. The complementary colour of the light absorbed
   D. Always black
   Answer: C. Remaining wavelengths are transmitted. (Key point: Observed colour)

4. Which ions are colourless?
   A. d5 ions
   B. d1 ions
   C. d9 ions
   D. d0 or d10 ions such as Sc3+ and Zn2+
   Answer: D. No d–d transition is possible when d is empty or full. (Key point: Colourless ions)

## Related in Engineering Chemistry

- [Why transition elements are coloured and paramagnetic](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-colour-paramagnetic) — Engineering Chemistry, chapter 5

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